The present invention relates to a process for forming a zinc phosphate coating on a metal surface and to a zinc phosphate coating composition. More specifically, it relates to a process, and composition useful therein, for forming on a metal surface a zinc phosphate coating that comprises an undercoat for the cationic electrodeposition of a paint film. The coating exhibits good corrosion resistance and adheres well to the paint film.
Generally, zinc phosphate coatings have been applied as an undercoat or primer for paint films applied by anionic electrodeposition so as to give better adhesion and corrosion resistance. The most common method of application is by spraying due to the cost of facilities and the efficiency of production.
Zinc phosphate coatings applied by spraying and suitable as an undercoat for anionic electrodeposition, as well as specific coating compositions for use therewith, are disclosed in many papers and patents. For example, Japanese Patent Publication No. 5086/1973 has proposed to add 0.5 to 8.0 mg/l of a copper ion to the usual zinc phosphate coating composition to reduce the rate of elution of the zinc phosphate coating during electrodeposition.
Japanese Patent Publication No. 34655/1973 has proposed to add to the known zinc phosphate coating composition 0.02 to 0.1g/l of an aluminum ion, 0.04 to 0.4 g/l of an arsenic ion and 0.02 to 2.0 g/l of a fluoride ion, together with 0.01 to 0.13 g/l of a nitrite ion as an oxidizer. According to the disclosure, the amount of the converted or formed coating dissolved during electrodeposition is reduced; the electrical conductivity is good; the appearance and rust resistance of the paint film are superior to those of the prior art; and contamination of the paint used in continuous application is remarkably reduced.
Japanese Patent Publication No. 6418/1975 has proposed to control the weight ratio of zinc to phosphoric radicals within the range of from 1:12 to 1:110, preferably from 1:20 to 1:100. This lower ratio of zinc to phosphoric radical provides thin, dense and even zinc phosphate coatings that contain a large proportion of iron resulting in a higher acid resistance. As a result, when the pH at the boundaries of the object being coated shifts to the acid side of the pH range during electrodeposition, the amount of elution of the undercoat decreases, thereby reducing the amount of the undercoat mixed into the paint film and the amount of reaction between them. This is said to remarkably reduce deterioration in the paint film.
Thus, with regard to undercoats for anionic electrodeposition, various improvements, including the addition of metal ions to the coating composition and control of the ratio of zinc to phosphoric acid in the coating liquid, have been proposed to provide coatings having excellent acid resistance and electrical conductivity.
In recent years, steel materials of which only one side is galvanized or alloyed with zinc have been used to enhance corrosion resistance after painting. The other side of such one-side galvanized steel, to which a trace of plating zinc adheres, requires buffing before use. Paints themselves are shifting from the anion type to the cation type. In particular, such a shift is being prompted in the automobile industry by a desire to minimize openings or holes in the underbody and rust on the body panels.
Various problems that can be encountered in the use of prior art zinc phosphate coating compositions in cationic electrodeposition applications are: the cationic electrodeposited films do not possess their normal properties, with the result that corrosion resistance is not much better than when iron phosphate coatings are used; adhesion is much worse than when iron phosphate coatings are used; and, in the "Checkerboard Test" after immersion in hot water, the entire film can come off. A good result is obtained only when the undercoat is treated with a chromic acid.
Our studies of differences between anionic and cationic electrodeposition and undercoats suitable for cationic electrodeposition have revealed the following:
First, as disclosed in Japanese Patent Disclosure No. 77144/1977, cationic electrodeposition paints, unlike anionic electrodeposition paints, are cured or hardened when the alcohol blocking the bridging agent (i.e. isocyanate) evaporates during baking. Thus, a large degree of contraction of the paint film occurs during baking, thereby exerting a considerable force on the undercoat. For this reason, it is necessary that the zinc phosphate undercoat for cationic electrodeposition, unlike that for anionic electrodeposition, possess sufficient strength to withstand contraction of the paint film.
Second, in cationic electrodeposition, the vicinity of the object being treated becomes alkaline so that it is necessary for the undercoat to have excellent alkali resistance. This is in contrast to anionic electrodeposition in which the vicinity of the object becomes considerably acidic.
Our studies of zinc phosphate coatings, compositions which form such coatings, and methods of applying such compositions have revealed that there is a strong correlation between achieving the aforementioned properties of strength and alkali resistance and the crystal structure of the resultant zinc phosphate coating. It has been observed that coatings formed from prior coating compositions have a leaf-like crystal structure (see Photograph 4), very poor adhesion to cationic electrodeposition paints, and very poor corrosion resistance. In contrast, the coatings formed according to the present invention have a substantially plate-like crystal structure (see Photograph 1) that does not grow in the vertical direction to the substrate, and this plate-like crystal structure exhibits both sufficient adhesion to the paint film and sufficient corrosion resistance after painting to serve as an undercoat for cationic electrodeposition paints. The coatings of the present invention, without treatment with chromic acid, have exhibited better adhesion and corrosion resistance than those prior zinc phosphate coatings treated with chromic acid.